The natural ageing of materials under sunlight is simulated in a known way by exposing samples to a source of electromagnetic radiation whose spectral distribution has to be carefully chosen to obtain accelerated ageing that can be correlated with the ageing observed under natural conditions. There are known devices comprising a fixed bank of low-pressure arc tube lights that provide ultraviolet radiation. This bank of parallel tubes is positioned so that it faces the samples to be tested.
The drawback of the radiation from low-pressure arc tubes is that there is a low emission and an ultraviolet spectrum that is very different from a solar spectrum. This makes the tests faulty. Another drawback is that these devices cannot be used to provide an even exposure of the samples. This is to the point where the test results have poor reproducibility.
There are known devices using xenon arc lamps whose value is that they have a spectrum very close to the solar spectrum when they are appropriately filtered. With recent developments in this technique, it is now possible to adopt filtered medium-pressure mercury vapor lamps whose spectrum is very rich in ultraviolet rays and shows a high equilibrium between the longer-wavelength U.V.A. type ultraviolet rays and the shorter-wavelength U.V.S. type ultraviolet rays. This enables an efficient reproduction of the ageing process caused by sunlight.
There are known instruments for carrying out ageing tests in which the samples are mounted on a vertical, cylindrical sample-holder rotating around vertically positioned tubular xenon arc lamps. The rotation of the samples about the xenon lamps provides for even exposure to radiation. However, these lamps release a lot of heat and it is necessary to plan for an open-circuit or a closed-circuit air circulation to maintain the samples at a controlled temperature.
These instruments have the drawback of exposing the samples to varying temperatures. Indeed, the ventilation circuit has an air suction zone above the cylindrical sample-holder that prompts a vertical air flow as in a chimney stack. The air flow is heated while moving vertically along the lamps and the samples, so that the samples positioned at the top of the sample-holder are at a temperature greater than that of the samples positioned at the bottom. Identical samples located at different heights are therefore subjected to different temperatures. This again affects the reproducibility of the tests. In practice, certain users are forced to interrupt the tests to switch the positions of the samples being tested, between the top and the bottom of the sample-holder.
Devices are also known with a parallelepiped-shaped test chamber having mercury lamps positioned vertically at the four corners of the chamber, along a small, rotating sample-holding cylinder placed at the center of the chamber. The lamps and the external face of the samples are cooled by ventilated air circulation with air inlets opened in the side walls of the chamber, and an air outlet capping the entire sample-holding cylinder at the top of the chamber.
One drawback of this device is that it takes up far too much space in relation to the small surface area of the samples that can be tested in it. This device also has the drawback of showing major losses of light energy, since more than 75 percent of the light is not sent directly to the sample-holder. This device therefore has low efficiency when the depreciation cost of the equipment added to the energy expenditure is related to the surface area of the samples being tested. This adversely affects the cost of the tests.
There also exist instruments comprising a rectangular test chamber in which there is mounted a cylindrical sample-holder cage mounted rotationally on a solid driving shaft. A laminar air flow travels along the samples, propagating vertically between apertures made in the axis of the generatrix lines of the cylinder at the floor and at the ceiling of the chamber to isolate the samples from the flow of hot air coming from the lamp and to provide common testing temperatures.
Despite the laminar air flow, this instrument still has the drawback of exposing the samples to different temperatures, as the laminar air flow becomes further heated when in contact with the samples exposed to a strong light radiation. In practice, the samples placed at the top of the cage are thus exposed to a temperature several degrees higher than that of the samples placed at the bottom of the cage.
European Patent No. 320,209 describes an atmospheric test cabinet or cubicle comprising a sample holder rack rotating about a xenon light tube positioned vertically. The samples are cooled by a laminar air flow that travels vertically along the internal walls of the samples positioned along the straight sides of the rack. For reasons of economy, the air flow has a limited flow rate. This test cubicle having a vertical laminar air flow and a low flow rate has the drawback of not providing for the efficient cooling of the samples and of inducing temperature differences between the samples placed at the bottom and those placed at the top of the rack.
U.S. Pat. No. 4,760,748 describes another instrument for the testing of accelerated ageing also comprising a cylindrical sample-holder frame mounted rotationally about the vertical axis, light tubes and with a rising laminar air flow cooling the internal face of the samples. The cylindrical frame has solid walls pierced with two rows of apertures to place the samples, and to form suction holes for a secondary air flow.
For the cooling, the instrument comprises an air circulation system with two air flows, with a rising main air column that flows up in the sample-holder cylinder traveling along the internal face of the samples, and a peripheral secondary air flow that strikes the external face of the samples. The secondary make-up air flow comes from a source of cold air. The secondary air flow is created by the suction effect prompted at the holes of the cylinder by the ascent of the main air column. The holes have a variable shuttering system to dose out the peripheral secondary air flow in relation to the rising main air flow.
The drawback of this device lies in the complexity and particularly large spacing requirement of its dual-flow air circulation system. Another drawback is that the testing space of the device and its capacity in terms of numbers of samples is quite restricted when related to the huge space requirement of the cubicle with its air-removal instrumentation at the top, the blower system and the rotational driving system at the lower part, as well as the peripheral air circulation system and the lateral cold air source. This dual-air-flow cooling system also has the drawback of giving rise to substantial temperature differences between the irradiated internal face and the cooled external face of the samples.
German Patent No. 3,243,722 discloses a device that is quite different for the testing of resistance to light and weather changes. This device comprises a large ventilated chamber in which there is positioned a horizontal assembly comprising a ring of light tubes inserted between two horizontal, concentric air conduits and a sample-holder drum mounted rotationally around horizontal conduits. The two conduits are connected to a fan and communicate with each other as well as with the outside of the chamber so that outside air is blown into the first conduit, then returns and is inserted into the space between the first and second conduits to along the light tubes for cooling them while returning to the exterior.
As for the samples positioned on the outer drum, they are cooled separately by another vertical air flow that circulates in a circuit provided with another fan and a cooling exchanger as well as, if necessary, heating means or a heater. This device therefore has two air circulation systems cooling first the samples, and then the light tubes.
In this device, the light tubes as well as a sprinkler system are positioned in conduits that separate them from the samples. This has the drawback of adversely affecting especially the exposure of the samples to light and the efficiency of the system. The device further has the drawback wherein the vertical air flow that crosses the sample-holder drum does not provide a desired cooling of the samples.